Pathogen diseases frequently occur in drought-stressed trees. Climatic extremes, such as drought, and associated shifts in pathogen dynamics are predicted to increase under climate change. However, few empirical studies address the combined impact of pathogenic infection and drought on trees’ physiology and the subsequent pathways to mortality.
We studied the combined effect of drought and infection by Heterobasidion annosum, which necrotrophically colonizes sapwood, on 46 Picea abies (Norway spruce) and 40 Pinus sylvestris (Scots pine) saplings. In August 2017, half of the saplings were inoculated inserting infected wood plugs in the trunk, while the other saplings served as control. Both inoculated and non-inoculated saplings were split in two watering regimes: to a gravimetric soil water content of 20% (drought) or 40% (control). The resulting treatment combinations were: Control-non-inoculated, Control-inoculated, Drought-non-inoculated and Drought-inoculated. From October 2017, saplings were monitored every two weeks for one year for gas exchange, needle relative water content and needle phenology. From December 2017, saplings underwent a 10-week simulated dormancy with no light and low temperatures. We used accelerated failure time models to assess survival across treatments and species. In spruce, we analyzed individual traits determining mortality, and timing of needle phenology across treatments implementing Bayesian models.
Results/Conclusions
We found a host-specific response to inoculation and drought treatments. Survival between drought-inoculated pine and spruce saplings was similar. However, survival and physiological response differed between species for inoculated- and drought-only treatments. Drought and/or inoculated pine showed reduced photosynthesis and stomatal conductance, while inoculated spruce maintained these at control levels. While 90% of inoculated pine saplings died before dormancy, inoculated spruce massively died one week after dormancy. Drought caused early mortality in pine saplings (62% before dormancy), while drought-treated spruce started to die during the flushing period after dormancy. Further, needle phenology in droughted spruce that survived was delayed by two-weeks compared to control and inoculated-only spruce.
Our results imply differential response to combined infection and drought in the two species. Inoculation worsened drought impacts in spruce, accelerating death, while the three treatments led to similar mortality rates in pine. These results highlight the fact that the interaction between drought and pathogen effects are likely to be highly species-specific. High post-dormancy mortality in inoculated spruce suggests lethal activity of the pathogen during our simulated mild winter. Higher impacts of co-occurring drought and pathogenic infection may occur on spruce under future climate change scenarios.